1. Field of the Invention
The present invention is directed generally to radio frequency (RF) transmitters and more particularly to power-efficient, small-packaged RF transmitters that have high gain, high power, and are required to be spectrally efficient.
2. Description of the Related Art
Radio frequency (RF) transmitters used for avionics applications, such as the VHF Data Link (VDL) Mode 2 application, are required to transmit at a high power with a high degree of spectral purity (also referred to as “spectral efficiency”). This means that they should transmit on-channel, with very little bleed-over of energy into adjacent channels. FIG. 1 is a graph 10 illustrating the spectral mask requirement for the VDL Mode 2 application as specified in the Minimum Operational Performance Standards (MOPS) and Test Procedures for Aircraft Very High Frequency (VHF) Digital Link (VDL) Mode 2 (DO#: DO-281A), hereinafter referred to as the “avionics specification” or simply the “specification.” The channel spacing for VDL Mode 2 is 25 kHz. This specification requires the active channel signal power to be a minimum of 15 watts with a minimum of 40 dB, up to 95 dB of suppression in the adjacent channels. Table 1 below shows a subset of the spectral mask requirements for adjacent channel power (ACP) according to the specification.
TABLE 1VDL Mode 2 Transmitter Adjacent Channel Power RequirementsFrequency OffsetPower (dBm)/25 kHz Power (dBc)/25 kHzOn-channel>+42n/a +/−25 kHz <+2<−40 +/−50 kHz<−28<−70 +/−75 kHz<−28<−70+/−100 kHz<−38<−80+/−300 kHz<−46<−88+/−500 kHz  <−49.6  <−91.6+/−800 kHz<−53<−95
The traditional solution to meet the spectral purity requirement has been to design a “Class A” transmitter architecture. Such as design is characterized by amplifiers that are biased in their linear region of operation. Although transmitters operating in a Class A configuration can have high spectral purity, their efficiency is very low. Typically, amplifiers operating as a Class A amplifier require a direct current (DC) bias that results in only about a 5% to 10% power efficiency. As an example, a Class A amplifier operating with an 8% efficiency that is required to deliver 15 watts of RF output power would require 187.5 watts of DC power (15 watts/0.08), and would dissipate 172.5 watts as heat.